Conjugate fibers having latent crimp that is developed into a spiral crimp, on account of shrinkage differences during a thermal treatment, are used, for instance, in stretchable nonwoven fabrics, high-cushioning nonwoven fabrics and liquid absorbing nonwoven fabrics. These conjugate fibers are made into webs mainly by way of a carding process, after which the spiral crimp is brought out in a thermal treatment in which the web shrinks to yield the nonwoven fabric. In the nonwoven fabric, therefore, the fibers are packed to a higher density than when in the web state, and are also entangled with each other on account of the spiral crimp. These features make for excellent stretchability, cushioning characteristics, as well as liquid absorption and release characteristics.
However, nonwoven fabrics obtained by way of a carding process have shortcomings in that the properties of the nonwoven fabric are not isotropic since the way in which fibers are arranged is different in the machine direction and the width direction. Patent document 1 discloses the feature of obtaining a nonwoven fabric, having substantial elastic recovery, by forming a latently crimp able conjugate fiber into a web, entangling the fibers using a water needle or the like, and developing then spiral crimp by way of a thermal treatment. In this nonwoven fabric, however, the fibers are arranged in the machine direction, and hence, although the nonwoven fabric exhibits excellent strength and elastic recovery in the machine direction, both strength and elastic recovery are remarkably small in the width direction.
An appropriately high existence density of fiber is particularly important in liquid absorbing nonwoven fabrics. To achieve a high-density nonwoven fabric, a low-density nonwoven fabric is ordinarily compacted using a high-temperature calendering roll. Alternatively, straight fibers having no crimp imparted thereto are made into a nonwoven fabric in accordance with a papermaking method. In these nonwoven fabrics, however, fibers are thermal fusion bonded each other existing excessively close. The resulting nonwoven fabric is hard, and the size of voids between fibers is insufficient, all of which often renders the nonwoven fabric unsuitable for absorbing and releasing liquids.
By contrast, nonwoven fabrics obtained by subjecting a web comprising the above-described latently crimpable conjugate fibers to a thermal treatment, to bring out thereby a spiral crimp that causes the web to shrink, are suitable for liquid absorption and release, and exhibit a somewhat high fiber existence density. Such nonwoven fabrics are preferably used in that, moreover, the size of the voids created by the spiral crimp affords good liquid absorption and release characteristics. The carding process itself, however, has limits as regards basis weight. For instance, it has proved impossible to obtain, stably and with good productivity, liquid absorbing nonwoven fabrics having a high basis weight of 500 g/m2 or more. In webs obtained by a carding process, the fibers exhibit considerable distribution as regards degree of freedom. Portions with a high degree of freedom result in higher density through shrinking, whereas portions with a low degree of freedom result in low density, on account of little shrinking. This tends to yield a nonwoven fabric of uneven texture derived from biased web shrinking. To solve this problem, it becomes then necessary to carry out a further process for interlacing fibers, by way of a water needle or the like, before causing the fibers to develop a spiral crimp by thermally treating the web, as disclosed in patent document 1. Such an approach detracts considerably from workability and productivity.
Air-laying processes are effective in order to overcome the above problems of property anisotropy and inadequacy for high-basis weight articles, by easily affording high-basis weight nonwoven fabrics boasting small differences in fiber arrangement in the machine direction and the width direction. However, there were problems that processability and productivity ordinarily became extremely low when latently crimp able conjugate fiber was applied to the air-laying process. Such poor processability and productivity are caused by the high bulk of latently crimp able conjugate fibers, the ready spreading of the fibers and the ready entangling of the spread fibers, which arise in turn from a considerable three-dimensional crimp shape, or planar but curved crimp shape, derived from the cross-sectional shape of the fiber.
Patent documents 2 and 3 disclose ways of obtaining high-bulkiness nonwoven fabrics by using, in an air laying process, latently crimpable fibers having a two-dimensional crimp such as zig-zag crimp or Ω-like crimp before a thermal treatment, such that the fibers develop a three-dimensional crimp after web formation. The crimp of the fibers is herein a two-dimensional crimp, such as zig-zag crimp or Ω-like crimp, with a view to improving air-laying processability. However, although the fibers developed a three-dimensional crimp by way of a thermal treatment, the crimp-eliciting power of the fibers was weak, and the web itself failed to shrink with high shrinkage. Therefore, the nonwoven fabrics exhibited low fiber existence density, insufficient three-dimensional isotropy, insufficient stretchability and cushioning characteristics, and insufficient liquid absorption characteristics. Moreover, the component used in the fibers was a polyester resin. This was problematic in that polyester resins have poor chemical resistance, and hence the nonwoven fabric was not suitable for alkaline liquids or the like, when used as a liquid absorbing nonwoven fabric.
Patent document 1: JP 02-127553 A
Patent document 2: JP 2003-166127 A
Patent document 3: JP 2003-171860 A